Spread spectrum modulated signals (CDMA, Code Division Multiple Access) are used, for example, in global navigation satellite systems (GNSS), such as the GPS (global positioning system) system, as well as in many third generation mobile communication systems, such as the UMTS (universal mobile telecommunication system). For generating a spread spectrum modulated signal, the modulation is performed in a transmitter by using an individual spreading code, wherein several transmitters can simultaneously transmit a signal at the same frequency, when each transmitter is allocated a unique spreading code. For example, in satellite positioning systems, each satellite uses a spreading code of its own. In the receiver, the corresponding reference code is generated or it is read from the memory of the receiver, and this reference code is used for searching the received signal for the signal of the transmitter which is to be received. For successful signal reception, the receiver must perform acquisition of the signal, typically by using several correlators and controlling the code phase and frequency of the reference code, wherein the signals generated by the correlators are used to determine the correct code phase and the frequency shift. After the acquisition has been completed, the tracking of the signal is continued so that the reception of the signal and the demodulation of the information transmitted therein would be possible. In this tracking step, the code phase and frequency of the reference code are to be kept locked with the code phase and frequency of the signal to be received.
In the acquisition phase it is desirable to cover as large a search range as possible, while in tracking phase the coverage is generally not an issue, but tracking accuracy requirements will usually need better timing resolution than what is required for the acquisition phase. This is usually achieved by changing the sampling frequency of the incoming signal. For acquisition phase, lower sampling frequency allows more time coverage in a given number of samples. For tracking phase, higher sampling frequency results in increased time resolution for improved tracking accuracy.
The acquisition and the tracking of the signal are problematic particularly indoors where the strength of the signal to be received is poor, possibly even lower than background noise. Such a situation occurs particularly in satellite positioning systems, in which the signal to be received is very weak upon arrival on the earth, and indoors this signal can further be attenuated by the walls of buildings. To cure this problem, solutions of prior art are aimed at implementing the receiver by providing it with a large number of correlators and by using a long integration time. At present, receivers may comprise as many as about 16,000 correlators. For the sake of comparison, it should be mentioned that the first portable GPS receivers only comprised 12 or even fewer correlators. The increase in the number of correlators naturally also means that the circuit board area required for implementing the correlators is significantly increased as well. Furthermore, this increases the power consumption of the receiver. Because of the higher power consumption, the heating of the device may also be increased.
Group Correlator is a code correlation device, which is optimized for reception of multiple signals at the same time. It utilizes time multiplexing to share some of the signal processing hardware amongst several different processing channels. The original group correlator structure is intended to be used at a single sampling rate at a time. Thus one group correlator would be needed for acquisition phase processing and one for tracking phase processing.
A GNSS receiver needs to perform both the acquisition and tracking operations for full functionality. Additionally, in normal operation some of the received signals are being searched for while other signals are being tracked. It would be desirable to be able to handle both cases with minimal hardware and control complexity.
Furthermore multiple GNSS systems and operating modes bring the need to use many sampling frequencies in a GNSS receiver designed to acquire and track signals from more than one GNSS system. Hardware should be minimized while allowing maximal flexibility of resource usage for the software. Different operating modes for the correlation should have minimal effect to the rest of the hardware to simplify hardware design. Group correlator architecture is desirable due to its versatility in both search and tracking mode operation, but it cannot cover multiple input sample rates in the traditional form.
In order to perform both signal acquisition and tracking, a GNSS receiver would need to do one of the following. One alternative is to use one group correlator for the acquisition and one group correlator for the tracking. However, this would increase the hardware size. Another alternative is to operate one group correlator using two different clock frequencies. This has the drawback that the control of the receiver is very complicated since all the processing after the group correlator would need to be adjusted accordingly. A third alternative is to use two group correlators, but time-multiplexing the subsequent processing i.e. the stages after the group correlator perform acquisition and tracking related operations in a multiplexed manner. This approach increases the size and makes the control of the subsequent processing very difficult as streams of samples with different sample rates would need to be processed with just one hardware block.
In many prior art GNSS receivers a correlator hardware which only comprises a few code delays per channel has been used to acquire and track the signal. This kind of receiver operates too slowly for current demands, since the correlators have very limited search ranges. Therefore, some approaches have been developed in which separate acquisition accelerator hardware is used for acquisition phase and the tracking phase is implemented by traditional correlator hardware.
Some solutions are also known which use correlator structures that can be configured as acquisition accelerators in one mode and as tracking correlators in one mode. However, they are not able to perform acquisition and tracking at the same time.
As examples of GNSS systems we mention here the GPS (Global Positioning System) and Galileo. The GPS is already operating globally while the Galileo system is under construction when this patent application is filed. However, the operating parameters for the Galileo system have already been defined, but it should be noted that the parameters may change. The operating parameters of both the GPS and Galileo are shown in Table 1 from which it can be seen that there are some similarities between them. It can also be seen from Table 1 that the acquisition to both GPS and Galileo system signals might not be equally efficient if the same correlation length were used.
TABLE 1GPSGalileoChip rate (MHz)1.0231.023PRN code length in chips10234092BOC code factor (samples per chip)12Total code search range in equiv. chips10238184Oversampling ratio (samples per chip)22Minimum sampling rate (MHz)2.0464.092Total code search range in samples204616368
The PRN (Pseudo Random Number) code parameters determine the requirements for the correlation part of the GNSS receiver. The chip rate, BOC (Binary Offset Carrier) factor and oversampling ratio determine the needed sampling rate. The code length, BOC factor and oversampling ratio determine the number of samples needed to cover full code uncertainty.